Implement playpen defaults
Implement playpen defaults

Avoids issues where we enable `editable` but forget to specify `editor`
(eg, https://github.com/rust-lang/rust-by-example/issues/963).  Since we
already include Ace editor, seems like we can just treat it as the
default.

azerupi/mdBook,azerupi/mdBook,azerupi/mdBook,azerupi/mdBook